1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine, and more specifically, to a technology of addressing clogging of an injection port of a fuel injection valve that is interposed in an exhaust path.
2. Description of the Related Art
A NOx (nitrogen oxide) absorption catalyst and a DPF (diesel particulate filter) are well known as devices for purifying exhaust from diesel engines.
The DPF collects PM (particulate matter) contained in the exhaust. It is also known to place an oxidation catalyst in the upstream of the DPF in order to remove the PM that is collected by and deposited in the DPF. In this method, the PM collected by the DPF is burned by sending fuel into the oxidation catalyst and raising the exhaust temperature.
The NOx absorption catalyst absorbs NOx contained in the exhaust in a lean atmosphere. According to a conventional method of regenerating the NOx absorption catalyst, the NOx absorption catalyst is supplied with a reducing agent such as fuel. The NOx absorbed by the NOx absorption catalyst is then desorbed, reduced and detoxified.
In order to supply fuel to the oxidation catalyst and the NOx absorption catalyst, a well-known method disposes a fuel injection valve (exhaust-side fuel injection valve) in the upstream of these catalysts, and injects the fuel into the exhaust path.
In this method, a fuel injection amount to be supplied by the exhaust-side fuel injection valve is set to an optimum amount based upon operation conditions, such as engine revolution speed. However, in respect of the exhaust purification system with the fuel injection valve located in the exhaust path, there is a case where soot contained in the exhaust clogs up an injection port of the exhaust-side fuel injection valve, which causes an error in an actual fuel injection amount in comparison with a preset fuel injection amount. Given this situation, a system has been developed, in which a pressure sensor is interposed in a fuel supply path through which fuel is sent to the exhaust-side fuel injection valve. Based upon the pressure detected by the pressure sensor, it is estimated whether the injection port is in a state where it is clogged beyond a tolerance limit (hereinafter, such a state is referred to as a valve clogging state). This system is disclosed in Japanese Patent No. 3514230 (hereinafter, referred to as Document 1).
On the other hand, the system described in Document 1 requires to newly install a pressure sensor for estimation of the valve clogging state. This increases components and raises costs. Moreover, if the pressure sensor does not operate properly, it is impossible to estimate the valve clogging state.